In the manufacture of semiconductor and in particular semi-insulating materials, e.g. in wafer form, there is a need for non-destructive testing thereof for defects in the material before relatively expensive electronic devices are built therefrom. These defects include dislocations and (in gallium arsenide, GaAs) EL2 defects. Clustering of defects, even of beneficial ones like EL2 (which makes GaAs semi-insulating) is undesirable. And as indicated above, useable electronic devices require high quality wafer material, which includes high uniformity across such material.
Infrared absorption has become a valuable technique for mapping the EL2 defect concentration in semi-insulating (SI) GaAs substrates. Two different methods are used. The first method, whole wafer mapping, is performed by illuminating the wafer with a lamp and photographing an image on the wafer's opposite side with, e.g. a camera having infrared sensitive film. This method is slow because it requires film processing. Although post-processing of the film image can be converted to quantitative maps of EL2, calibration of the film and light source is necessary, resulting in further delays and possible loss of image fidelity.
Another type of whole wafer mapping is performed by illuminating the wafer with a lamp and obtaining an image on the wafer's opposite side with a vidicon camera. This type of whole wafer mapping provides only qualitative data because the vidicon camera is neither sensitive nor uniform enough to provide truly quantitative images.
The second method is to use lenses to focus the light to a spot on the wafer about 1 mm in diameter and measure the transmission of light through the wafer at successive (1 mm wide) points, by moving the spot across the wafer or vice versa. Such spot scanning method provides a point-by-point EL2 concentration map, but typically requires about two hours to set up and scan each wafer.
For examples of spot scanning patents, see U.S. Pat. No. 5,126,569 to Carlson (1992) and U.S. Pat. No. 4,578,584 to Baumann (1986), which references are incorporated herein by reference.
For another example of spot scanning in the prior art see an Article by D. C. Look, et al. A new technique for whole-wafer etch-pit density mapping in GaAs, "J.Appl. Phys." 65 (3), 1375, Feb. 1, 1989, which Article is incorporated herein by reference. Also the spot scanning method can leave significant blanks in the image obtained, e.g. where the defect density varies rapidly within each spot.
Thus, there is need and market for a solid material defect mapping system that overcomes the above prior art shortcomings. There has now been discovered a fast, accurate, and non-destructive means of imaging defects and determining defect densities in solid materials. The method of the invention is highly suitable for obtaining accurate images and maps of concentration of defects, e.g. in whole semiconductor wafers or in either large or small portions of such wafers.